The following U.S. Patents are believed to represent the current state of the art: U.S. Pat. Nos. 6,809,724; 6,798,391; 6,771,423; 6,757,068; 6,747,611; 6,727,865; 6,681,629; 6,661,438; 6,608,884; 6,563,648; 6,559,872; 6,474,159; 6,408,257; 6,384,982; 6,380,923; 6,359,609; 6,356,392; 6,353,503; 6,304,303; 6,222,677; 6,204,974; 6,157,533; 6,140,981; 6,127,990; 6,108,197; 6,094,242; 6,057,966; 6,050,717; 5,886,822; 5,880,773; 5,844,656; 5,798,733; 5,764,280; 5,581,492 and 5,757,339.
The following patent publications are also believed to be of interest:
Published PCT Applications: WO04015369; WO02086590; WO0180736; WO0156007; WO0116929; WO0109636; WO0023815; WO0010156; WO9733270; WO9725646; WO9637798; WO9636898; WO9607947; WO9605532; WO9600406; WO9521395; WO9510106; WO9510061; WO9424658; WO9414152; WO9411855; WO9407161 and WO9301683.
Foreign Patent Publications: EP1310859; EP1280457; EP1267197; EP1248180; EP1223729; EP1220080; EP1185142; EP1176449; EP1135765; EP1042698; EP1022644; EP0935183; EP0904560; EP0902312; EP0889346; EP0825470; EP0802440; EP0775327; EP0772790; EP0771433; EP0724758; EP0721614; EP0716329; EP0694196; EP0670537; EP0672286; EP0627644; EP0592591 and EP0344881.